The present invention relates to an electrode sheet for an electric double-layer capacitor, a method for its manufacturing, a polarizable electrode and an electric double-layer capacitor using the polarizable electrode.
An electric double-layer capacitor, which has high capacitance of farad class, excellent characteristics of charge/discharge cycle and the capability of undergoing rapid charge, has been used for a backup power supply of electronic component, an onboard battery for a vehicle (an energy buffer) and the like.
An electric double-layer capacitor is briefly described referring to FIG. 1.
FIG. 1 is a sectional view showing the main structure of an electric double-layer capacitor.
As shown in FIG. 1, an electric double-layer capacitor 101 includes a casing 102 housing a pair of carbon electrodes (polarizable electrodes) 104 which interposes a separator 103, and a pair of collectors (elements) 105. And the casing 102 is filled with an ion conductive electrolytic solution. The electric double-layer capacitor 101 employs electric charges (shown by + and xe2x88x92 in FIG. 1) as dielectrics in a normal capacitor, which are generated at an interface between the solid carbon electrodes 104 and the liquid electrolytic solution, and spaced at a distance of molecule.
Electrolytic solutions used for electric double-layer capacitors are roughly categorized into an aqueous electrolytic solution made of a dilute sulfuric acid added with an electrolyte and an organic electrolytic solution made of an organic solvent added with an electrolyte. An appropriate type of electrolytic solution is selectably applied to an electric double-layer capacitor taking into account its usage. An electric double-layer capacitor using an aqueous electrolytic solution is advantageous in terms of lower internal resistance and higher power density. On the other hand, an organic electrolytic solution, which allows a higher withstand voltage per cell, is advantageous in terms of energy density. It also allows selection of inexpensive and light metals such as an aluminum alloy for a casing.
A method for manufacturing an electrode sheet is generally used, which has a process of mixing an electrochemically active material such as activated carbon, an electrically conductive filler such as carbon black and a binder such as polytetrafluoroethylene (PTFE). Japanese Patent Publication 07-105316 (Japanese Published Patent Application 63-107011) reports the related art.
Japanese Patent Publication 07-105316 does not fully explain the interaction between an electrode sheet and an electric double-layer capacitor, although the document introduces the parameters such as the porosity and Gurley number of electrode sheet. This has necessitated establishment of a criterion, which is applicable to manufacturing of an electrode sheet. Especially, the criterion for electrode sheet, which is used for an electric double-layer capacitor that has low electrical resistance and stable performance, has been in great demand.
In the circumstances, the present invention provides an electrode sheet and its manufacturing method, which enable manufacturing of this type of electric double-layer capacitor. The present invention also provides a polarizable electrode including the electrode sheet and the electric double-layer capacitor using the polarizable electrode.
Applicants have found that a contact angle between an electrode sheet and a droplet of electrolytic solution can be controlled so that an electric double-layer capacitor can have low electrical resistance and stable performance. The applicants have also discovered that the desirable contact angle can be obtained by manufacturing the granules for electrode sheet under preferably selected conditions of process associated with kneading.
An aspect of the present invention provides an electrode sheet for an electric double-layer capacitor, and the electrode sheet is molded from granules which are produced from ingredients including an electrochemically active material, an electrically conductive filler and a binder. In this aspect, a contact angle is equal to or less than 100 degrees when the contact angle is defined as (180-ALPHA) degrees, where ALPHA represents an apex angle of a droplet of an electrolytic solution for the electric double-layer capacitor, and when the droplet lies on the electrode sheet.
In this way, it is possible to obtain the electrode sheet described above, which can be used for the electric double-layer capacitor having stable performance and low electrical resistance due to improvement in wetness.
Another aspect of the present invention provides a method for manufacturing an electrode sheet for an electric double-layer capacitor. The electrode sheet is molded from granules which are produced from ingredients including an electrochemically active material, an electrically conductive filler and a binder. The method includes the steps of (a) kneading the ingredients so that the binder is subjected to fibrillation, and molding a lump out of the ingredients after the fibrillation, (b) crushing the lump into granules for the electrode sheet of the electric double-layer capacitor and (c) forming the granules into the electrode sheet. In this aspect, one of a period of time and strength of kneading at the step (a) is adjusted so that a contact angle can be equal to or less than 100 degrees when the contact angle is defined as (180-ALPHA) degrees, where ALPHA represents an apex angle of a droplet of an electrolytic solution for the electric double-layer capacitor, and when the droplet lies on the electrode sheet.
The electrode sheet described above allows repetitive manufacturing of an electric double-layer capacitor that has lower electric resistance and stable performance.
In this connection, the period of time and the strength correlated with the contact angle less than (180-ALPHA) degrees have been demonstrated by an experiment or a simulation based on data obtained by the experiment. The conditions applied to the experiment, which include ingredients, a mixing ratio, a thickness of electrode sheet and a method of manufacturing, are fixed for the experiment and simulation.
It is preferable to select classified granules having a particle diameter of 47-840 micron meters for manufacturing the electrode sheet according to the present invention.
The granules selected this way prevent fine particles of carbon and electrically conductive filler from separating or desorbing from the electrode sheet, so that they may not float in an electrolytic solution or exist between separators. It will enable manufacturing of the electric double-layer capacitor, which has high efficiency in terms of voltage maintenance.
Still another aspect of the present invention provides a polarizable electrode for an electric double-layer capacitor, including an electrode sheet and a collector foil. The electrode sheet is molded from granules which are produced from ingredients, including an electrochemically active material, an electrically conductive filler and a binder. The collector foil is bonded with the electrode sheet directly or via a layer of an adhesive so as to form the polarizable electrode. In this aspect, a contact angle is equal to or less than 100 degrees when the contact angle is defined as (180-ALPHA) degrees, where ALPHA represents an apex angle of a droplet of an electrolytic solution for the electric double-layer capacitor, and when the droplet lies on the electrode sheet.
The polarizable electrode for the electric double-layer capacitor described above has high efficiency in terms of voltage maintenance as well as the electrode sheet according to the present invention.
Yet another aspect of the present invention provides an electric double-layer capacitor including the polarizable electrode described above.
This electric double-layer capacitor has not only low electric resistance but also stable performance.